The present invention relates to organoaluminum electrolytes suitable for the electrolytic deposition of aluminum or aluminum-magnesium alloys on electrically conductive materials, and a method for this using soluble aluminum anodes or soluble aluminum and magnesium anodes or an anode made of an aluminum-magnesium alloy.
Organoaluminum complex compounds have been used for a long time for the electrolytic deposition of aluminum (dissertation H. Lehmkuhl, TH Aachen 1954, German Patent 1047450; K. Ziegler, H. Lehmkuhl, Z. anorg. allg. Chemie 283 (1956) 414; German Patent 1056377; H. Lehmkuhl, Chem. Ing. Tech. 36 (1964) 616; EP-A-0084816; H. Lehmkuhl, K. Mehler and U. Landau in Adv. in electro-chem. Science and Engineering (Eds. H. Gerischer, C. W. Tobias, Vol. 3, Weinheim 1994). As suitable electrolytes, there have been proposed complex compounds of general type MX2AlR3, which are employed either as molten salts or in the form of their solutions in liquid aromatic hydrocarbons. MX may be either alkali metal (Na, K, Rb, Cs) or onium halides, preferably fluorides. R represent alkyl residues with preferably one, two or four carbon atoms.
The interest in electrolytic coatings of metal workpieces with aluminum has greatly increased due to the excellent corrosion protection by the aluminum layers and their ecological safety. Therefore, the galvanic coating with organoaluminum electrolytes which work at moderately elevated temperatures of between 60 and 150xc2x0 C. and in closed systems is of great technical importance.
Since it has been sought, in recent years, to develop motor vehicles optimized in terms of consumption and weight, a consequent light-weight construction more and more requires the use of aluminum or magnesium or their mutual alloys. However, the light metal materials have a drawback in that both aluminum and magnesium have a high solution pressure in aqueous medium. Mainly upon contact with steels or conventionally galvanized steels, there is contact corrosion. For this reason, it is required to coat fixing members on magnesium applications in such a way that contact corrosion on the magnesium is avoided, on the one hand, and a long-term stability of the coating is obtained, on the other hand. The galvanic coating of the connecting screws with aluminum alone serves this function only partially since the corrosion products of the construction material magnesium are alkaline and attack the aluminum surfaces of the coating (B. Reinhold, S. G. Klose, J. Kopp, Mat.-wiss. u. Werkstofftech. 29, 1-8 (1998).
Methods for the galvanic deposition of aluminum-magnesium alloys on electrically conducting materials are known: J. H. Connor, W. E. Reed and G. B. Wood, J. Elektrochem. Sc. 104, 38-41 (1957), describe only briefly that they obtained a metal layer with 93% Al and 7% Mg having a good appearance upon electrolysis of AlBr3, Li[AlH4], MgBr2 (Mg/Al=0.8) in diethyl ether. J. Eckert and K. Gneupel obtained metal depositions with up to 13% Mg from a similar electrolyte of AlCl3, Li[AlH4], MgBr2 in a mixture of THF, diethyl ether and benzene (Mg/Al=0.6) (GDR Patent Specification 244573 A1). The conductivity of the electrolyte was on the order of 1.10xe2x88x923 to 7.10xe2x88x923 S.cmxe2x88x921. In the GDR Patent Specification 243723 A, the same authors describe an electrolyte solution consisting of ethylmagnesium bromide and triethylaluminum in THF/toluene 1:1 from which metal layers with a maximum of 10% Al were obtained.
Typical electrolytes, which have also proven technically useful for the deposition of aluminum, based on organoaluminum complex compounds of the type M[R3Alxe2x80x94Xxe2x80x94AlR3] (R=Et, iso-Bu; X=F, Cl; M=K, Cs, N(CH3)4) have been used for the electrochemical deposition of aluminum-magnesium alloys and magnesium by A. Mayer, J. Electrochem. Sci. 137 (1990), and in the U.S. Pat. No. 4,778,575 (priority of Oct. 18, 1988) after the addition of trialkylaluminum (R=Et, i-Bu) and dimethyl- or diethylmagnesium.
However, in a technical application of this method, the following problems arise, which render a continuously operating coating process impossible.
In contrast to aluminum anodes, magnesium anodes cannot be dissolved in the coating process with the proposed electrolytes. Continuous replenishing of the Mg content by dissolving the magnesium anode is not possible using organoaluminum complexes containing fluoride or generally halide as electrolytes.
According to the description in the U.S. Pat. No. 4,778,575, dialkyl magnesium in ethereal solution is employed for preparing the electrolyte. In a continuously working coating method, the dialkyl magnesium would have to be fed constantly in ethereal solution. However, diethyl ether is known to cleave some complexes, e.g., Na[Et3Alxe2x80x94Fxe2x80x94AlEt3] into Na[Et3AlF]+Et3Al.OEt2 (K. Ziegler, R. Kxc3x6ster, H. Lehmkuhl, K. Reinert, Liebigs Ann. Chem. 629, 33-49 (1960)). If the use of ether as the solvent for dialkyl magnesium was to be avoided, dialkyl magnesium would first have to be rendered ether-free, which requires considerable expenditure and costs, or it would have to be prepared in an ether-free form by the reaction of magnesium metal with di-alkylmercury, a very toxic compound.
For the reasons already described, it has been the object of the present invention to provide halide-free organoaluminum electrolytes which combine in themselves optimally the properties required for a technical application for the deposition of aluminum and aluminum-magnesium alloys, such as solubility of both aluminum and, in the case of alloy layers, magnesium anodes by electrolysis, as high as possible a conductivity, homogeneous solubility in aromatic solvents, such as toluene at between 20 and 105xc2x0 C., cathodic deposition of dense layers of aluminum-magnesium alloys with selectable proportions of the two components of from Al:Mg=95:5 to 5:95.
The object has been achieved by the use of organoaluminum electrolytes which are characterized by containing either (in the case of electrolyte type I) alkali tetraal-kylaluminate M[AlR4] or (in the case of electrolyte type II) alkali hexaalkylhydrido-dialuminate and additionally M[AlR4] as well as trialkylaluminum AlR3 (R=CH3, C2H5, C3H7 or n- or iso-C4H9; M=Li, Na, K, Rb, Cs), while electrolytes of composition M[R3Alxe2x80x94Hxe2x80x94AlR3] have proven particularly useful for the preparation of pure aluminum layers.
For reasons of optimizing solubility, specific conductivity and good accessibility, the ethyl compounds (R=C2H5=Et) are preferred. An electrolyte according to the invention of type I is dissolved in 2.5 to 6 mol per mole of complex compound of an aromatic hydrocarbon liquid at 20xc2x0 C., preferably in toluene or a liquid xylene. The trialkylaluminum is preferably triethylaluminum (AlEt3), and alkali tetraalkyl-aluminate is preferably a mixture of potassium and sodium tetraethylaluminates. The quantitative ratio of complex : AlEt3 is from 1:0.5 to 1:3, preferably 1:2. The proportion of Na[AlEt4] is between 0 and 25 mole percent, based on the total amount of K[AlEt4] and Na[AlEt4], but preferably between 5 and 20 mole percent. The addition of low amounts of Na[AlEt4] is preferred because, when this component is lacking, the aluminum anodes are dissolved only with moderate to poor current efficiencies, e.g., only about 22% in K[AlEt4]/3AlEt3/6 toluene, which would lead to a loss of triethylaluminum for extended durations of the electrolysis. The electrolysis is performed at temperatures of between 80 and 105xc2x0 C., preferably between 90 and 100xc2x0 C.
An illustrative electrolyte I is 0.8 mol of K[AlEt4]/0.2 mol of Na[AlEt4]/2.0 mol of AlEt3/3.3 mol of toluene. From this electrolyte solution, there is no crystallization even upon extended standing at room temperature, and the specific conductivity at 95xc2x0 C. is 13.8 mS.cmxe2x88x921.
The addition of at least 0.3-0.5 mol of triethylaluminum is necessary to avoid the deposition of alkali metal during the electrolysis. The addition of larger amounts of AlEt3 (2-3 mol AlEt3 per mole of complex) has a very positive effect on the alloy deposition; the alloy layers obtained thereby have 5-50% by weight of Mg, are very uniform, have a silky gloss and are essentially pore-free at a layer thickness of as low as 4-6 xcexcm. However, if the amount of triethylaluminum per mole of complex is increased from 2:1 to 3:1, it is required, in order to maintain a solution which is homogeneous even at room temperature, to add additional solvent to the electrolyte, i.e., to a total of 5.5-6 mol of toluene per mole of complex. However, the electrolyte loses conductivity thereby.
Electrolytes of type II preferably consist of mixtures of Na[Et3Alxe2x80x94Hxe2x80x94AlEt3], Na[AlEt4] and AlEt3. Despite of unfavorable properties of individual components, e.g., a relatively high melting point of Na[AlEt4] of 125xc2x0 C. and low solubility in toluene at 20xc2x0 C., mixtures of the three components with a suitable mixing ratio (molar ratio Na[Et3Alxe2x80x94Hxe2x80x94AlEt3] to Na[AlEt4] of between 4:1 and 1:1, preferably 2:1) are homogeneously soluble in toluene at 20xc2x0 C. and then have the properties required for a technical application for the deposition of aluminum-magnesium alloy layers, such as the solubility of both aluminum and magnesium anodes by electrolysis, as high as possible a conductivity, homogeneous solubility in aromatic solvents, such as toluene at between 20 and 105xc2x0 C., cathodic deposition of dense layers of aluminum-magnesium alloys with selectable proportions of the two components of from Al:Mg=95:5 to 5:95. Due to the presence of AlEt3, aluminum metal is deposited electrolytically from Na[AlEt4] rather than sodium metal (W. Grimme, dissertation TH Aachen (1960); DBP 1114330 (1959); DBP 1146258 (1961)). During electrolysis, Na[AlEt4] dissolves both aluminum and magnesium anodes (W. Grimme, dissertation TH Aachen 1960; K. Ziegler, H. Lehmkuhl, in Methoden der Organ. Chem. (Houben-Weyl), Vol. 13, 1, p. 281 (1970).
Electrolytes of composition M[R3Alxe2x80x94Hxe2x80x94AlR3] (M=Na, K, Rb, Cs; alkyl residue R=CH3, C2H5, C3H7, C4H9), e.g., Na[Et3Alxe2x80x94Hxe2x80x94AlEt3], as solutions in toluene are very highly suitable for the electrolytic deposition and dissolution of aluminum at 90-105xc2x0 C. However, we have found that magnesium anodes are not dissolved in the electrolysis of this compound in the absence of Na[AlEt4] according to the invention. After a current flow of 8.7 mF, the simultaneous use of an aluminum and a magnesium anode resulted in a weight loss of 8.7 meq of aluminum while the magnesium anode remained completely undissolved. This means that Na[Et3Alxe2x80x94Hxe2x80x94AlEt3] without Na[AlEt4] component represents an excellent electrolyte for the deposition of pure aluminum. However, for the preparation of aluminum-magnesium alloy coatings, the combination of both Na complexes with triethylaluminum and toluene has the effects
a) that the solubility of NaAlEt4 is sufficiently increased; and
b) that both aluminum and magnesium anodes are dissolved in this electrolysis mixture.
The electrolyte II according to the invention is dissolved in 5-7 mol per mole of Na[AlEt4] of an aromatic hydrocarbon liquid at 20xc2x0 C., preferably toluene or a liquid xylene. The quantitative ratio of Na[Et3Alxe2x80x94Hxe2x80x94AlEt3] to Na[AlEt4] is preferably 2:1 to ensure homogeneous solubility in 6 mol of toluene per mole of Na[AlEt4], and the molar ratio of Na[AlEt4] to AlEt3 is preferably 1:2 to ensure perfect metal deposition by electrolysis. An illustrative electrolyte II is 1 mol of Na[Et3Alxe2x80x94Hxe2x80x94AlEt3]/0.5 mol of Na[AlEt4]/1 mol of AlEt3/3 mol of toluene. Even upon extended standing at room temperature, there is no crystallization from this electrolyte solution which would interfere with the technical applicability of the electrolyte. Its specific conductivity at 95xc2x0 C. is 8.12 mS.cmxe2x88x921.
The electrolytic deposition of aluminum-magnesium alloy layers from the electrolytes according to the invention is performed by using a soluble aluminum anode and a similarly soluble magnesium anode, or by the use of an anode made of an aluminum-magnesium alloy. In the case of two anodes, to ensure a continuous operation and for controlling to obtain a selectable and desired alloy composition, the two anodes are separately connected. The electrolyses are performed in toluene solution, conveniently at 90-100xc2x0 C. The anodic (Al 95-100%; Mg 93-100%) and cathodic current efficiencies are practically quantitative. Since a finite and thus necessary concentration of magnesium in the electrolyte builds only in the course of the electrolysis, this condition must be brought about first before a freshly prepared electrolyte is employed. This can be done
1. by a short preliminary electrolysis during which the magnesium content in the cathodically deposited layer increases with increasing magnesium concentration in the electrolyte solution until the time when a suitable and desired selection of the anodic partial current densities causes that as much aluminum and magnesium is anodically dissolved as is cathodically deposited; or
2. by the addition of the complex compound Mg[AlEt4]2, a colorless liquid (K.
Ziegler, E. Holzkamp, Liebigs Ann. Chem. 605, 93-97 (1957)) which may also be used as a solution in toluene. After the addition of 0.01 mol of Mg[AlEt4]2 per 3.0 mol of K[AlEt4], for example, electrolyte I can be used directly for the coating according to the inventive method.
The electrolytic deposition from the electrolytes according to the invention yields aluminum-magnesium alloy layers which are clearly different from previously known layer systems in terms of their electrochemical properties. In the cathodic partial reaction, the electrochemical behavior of the alloy layers corresponds to the magnesium type, and in the anodic partial reaction, it corresponds to the aluminum type, associated with a pronounced passivity interval.
At room temperature in a 5% aqueous NaCl solution at a pH value of 9.0, the alloy layers have an open-circuit potential of about xe2x88x921380 to xe2x88x921500 mV versus S.C.E. at Mg incorporation rates of from 5 to 50% by weight. Due to the layer passivity (formation of intermetallic phases), the cathodic partial reaction is additionally inhibited upon contact with more electronegative metals, such as magnesium. The potential of the cathodic partial reaction is thereby shifted towards even more negative potential values as compared to the open-circuit potential. As a consequence, the remaining potential difference between the cathodic partial reaction of the alloy layer (at pH 9: oxygen reduction) and the anodic partial reaction of the magnesium is highly reduced. Thus, the AlMg alloy layers enable substantial adaptation to the open-circuit potential of the magnesium alloy AZ91hp, which is about xe2x88x921680 mV versus S.C.E., and the contact corrosion at the magnesium is highly reduced. Therefore, the alloy layers are suitable for the coating of steel fixing members in contact with magnesium. Potential applications include, in particular, applications in the automobile industry in the gear, engine and car body fields.
In addition, the alloy layers developed, which are deposited from non-aqueous electrolytes, are suitable as high quality surface coatings for highly heat-treated steel parts whose tensile strength is  greater than 1000 MPa and which cannot be coated with conventional galvanic methods due to the risk of hydrogen brittleness. Thus, there is a potential field of applications for the coating of heat-treatable and spring steels with alkali-resistant coatings compatible with aluminum and magnesium.